Fix xUnit discovery race in demo smoke tests

[Aaronontheweb]

Summary

• replace xunit.v3 3.2.2 with the fixed xunit.v3.mtp-off 4.0.0-pre.128 only in the Demo AppHost integration-test project
• preserve the existing VSTest execution path and avoid a repository-wide MTP migration
• document the upstream assembly-info JSON corruption race fixed after 4.0.0-pre.116

Root cause

The Windows CI failure matches xunit/xunit#3576 exactly: xUnit 3.2.2 can append Waiting 10 seconds for foreground threads to exit... to the assembly-info JSON consumed by the Visual Studio adapter, causing nondeterministic discovery failures. This is a runner protocol race, not a test timeout.

Validation

• 100/100 direct -assemblyInfo invocations emitted exactly one valid JSON object
• affected test project restored, built, and discovered successfully
• opt-in smoke test executed and skipped correctly without NETCLAW_RUN_DEMO_SMOKE=1
• dotnet slopwatch analyze passed with 0 issues
• pwsh ./scripts/Add-FileHeaders.ps1 -Verify passed
• git diff --check passed

No timeout values, sleeps, or retries were added.